This invention relates to apparatus for washing solid fractions such as screenings and grit extracted from the effluent flow of a sewage treatment plant to remove contamination by fecal solids and so render such fractions suitable for re-use or disposal.
It is recognized that materials entering a sewage treatment plant contains solid materials such as rags, paper, polythene and other plastic sheeting and the like which cannot be processed by the treatment plant. Screens or sieves capture such solids from the flow entering the sewage treatment plant and are cleaned periodically, or continuously, to remove the captured screenings for disposal. In order to minimize potential health hazards, and to improve the working environment of personnel handling the screenings, it is desirable that the screenings are free from fecal solids. Inevitably fecal solids from the flow entering the sewage treatment plant become entrapped with the screenings and the usual method of `cleaning` the screenings involves the total maceration of everything removed from the flow by the screens. During this process fecal solids are reduced in size to a larger extent than the screenings, and can thus pass through additional fine secondary screens to return to the main sewage flow, the macerated screenings being retained. It will be recognized however that maceration of all solids removed by the primary screens absorbs large amounts of energy. In addition some of the screenings will be reduced to a sufficiently small size as to pass through the secondary screens and thus some of the screenings join the main flow re-entering the sewage treatment plant. Moreover stones, and other hard objects can be carried by the flow and can be delivered to the maceration plant with the screenings. Such objects can seriously damage the cutting blades of the macerater and thus maceration to permit removal of fecal solids is expensive both in energy costs, and machinery maintenance costs.
A further disadvantage of maceration of the screenings is that it is generally more convenient to handle screenings for disposal in their whole state since this leads to easier compaction and de-watering of the screenings. However, maceration of fecal solids is advantageous since it liquifies or disintegrates the biodegradable solids thereby increasing their effective surface area and accelerating the subsequent biological treatment process.